1. Field of the Invention
The present invention concerns the hardening of tools with the aid of hard wear-resistant coatings. The tools considered are metal-removing tools, particularly rotary shaft tools such as drill bits, countersinks, screw taps, reamers, etc. for processing of metals in which metal is removed. The wear-resistant coatings are roughly 1 to 10 μm thick hard material coatings which are preferably deposited on the tool surface using physical vapor deposition (PVD).
2. Description of Related Art
The technical community has been concerned for some time now with finding a wear-resistant coating for the dry machining of metals. In this context, metal removal without any cooling agent or lubricant, but also metal removal with a minimal quantity of lubrication, is designated as dry, stable processing.
In developing the coating and particularly in selecting the coating material, an underlying consideration was that the tool assumes substantially higher temperatures during dry machining and that this undesirable temperature increase can be reduced if the largest possible share of the heat is carried away not via the tool but rather via the chips. It was considered accordingly to combine materials which were known either for a high elevated temperature hardness and/or a high oxidation stability and/or a low thermal conductivity.
The most common wear-resistant coating consists of golden-yellow titanium nitride, TiN. TiN coatings have universal applications. Coatings made of the dark blue-red lustrous titanium aluminum nitride (Ti,Al)N are known for their high elevated temperature hardness. They mostly have a percent ratio of titanium to aluminum atoms of 50:50 or rather (Ti0.5, Al0.5)N, which is occasionally also shifted in the direction 40:60 or rather (Ti0.4, Al0.6)N. In the hardening of tools, they have applications as a single layer coating (see, e.g., Gilles et al., Surface and Coatings Technology 94–95 (1997) 285–290) as well as a multilayer (Ti,Al)N/TiN coating with intermediate layers made of titanium nitride (see, e.g., the so-called FIRE coating by Gühring oHG).
CrN coatings are recommended for processing of nonferrous metals (see, e.g., P. Hones, Surface and Coatings Technology 94–95 (1997) 398–402).
Also known are MeCrAlY alloys (Me=metal) for coating turbine blades. They increase the oxidation stability and the thermal insulation and thus the allowable temperature and the efficiency of airplane engines (see, e.g., W. Brandl et al., Surface and Coatings Technology 94–95 (1997) 21–26).
Recently, a multilayer coating made of (Ti,Al)N and CrN has become known (see, e.g., I. Wadsworth et al., Surface and Coatings Technology 94–95 (1997) 315–321). Its oxidation stability grew as the Cr share was increased, at least up to Cr shares of 30%. In the same laboratory, coatings made of TiAlN with minor additions of Cr and Y were also studied (patent document DE 19818782, application date Apr. 27, 1998).